Method and apparatus for the purification of fused salt baths



R. J. MCNITT 2,414,831

CATION OF' FUSED SALT BA'I'HS` Jan. 28, 1947.

METHOD AND APPARATUS FOR THE PURIFI Filed Oct. l5, 1941 2 Sheets-Sheet 1 R. J. MCNITT Jan. 28, 1947.

METHOD AND APPARATUS FOR THE PURIFICATION 0F FUSED SALT BATHS Filed Oct. l5, 1941 2 Sheets-Sheet 2 r f a a n an J/ Ww M www 7 J @l A ATTORNEYS Patented Jan. 28, 1947 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR THE PURI- FICATION OF FUSED SALT BATHS Robert J. McNitt, Perth Amboy, N. J.

Application October 15, 1941, Serial No. 415,093

6 Claims.

This invention relates to the electrolysis of fused baths in which soluble oxygen compounds constitute a harmful impurity in the electrolyte. In these processes the impurity is decomposed with the liberation of oxygen or other harmful substances which may attack and corrode the active surface of the anode with an increase in the cell voltage and the consumption of electric energy, or may form soluble secondary substances which are in turn decomposed by the electric current thus opening the way to a cycle which causes a reduction in the current eiliciency to a small part of that which it is possible to maintain when the electrolyte does not contain such impurities. These impurities may enter the electrolyte during the preparation of the initial fused bath when a new cell is placed in operation, or they may be introduced with the replenishing substances introduced into the bath to replace those removed by the electrolysis. They may come from materials used in the construction of the various parts of the cell or from exposure of the upper surface of the bath to the air.

It is one aim of this invention to remove these soluble oxygen-bearing impurities from the fused electrolyte in the cell.

Another aim of the invention is to prevent such harmful impurities from entering the electrolyte with the replenishing materials.

The invention comprises circulating the electrolyte containing the dissolved oxygen compound into a zone of electrolytic action provided by purifying electrodes which decompose the oxygen compound and set free the oxygen. The purifying electrodes preferably comprise one or more carbon anodes maintained under such conditions that the carbon reacts with the oxygen forming an Oxy-carbon gas which escapes from the electrolyte.

The invention, in one of its applications, comprises passing electrolyte from the cell into a feed compartment and feeding into said electrolyte materials to replenish the bath. To avoid the possibility of admitting oxygen with the replenishing materials in a form which cannot be eliminated by submission to electrolytic action between the purifying electrodes, it is advantageous to give the materials in the feed compartment a preliminary treatment with a scavenging agent which combines with the oxygen, forming compounds which are soluble and insoluble and which may be separately removable from the bath. The soluble compounds are capable of electrolytic decomposition with the elimination of the oxygen through reaction with the carbon of the purifying anode. The insoluble compounds may be eliminated by settling and removal from the bath by a ladle.

In using purifying electrodes according to the invention, it is important that conditions are suitable for the reaction of oxygen with the carbon of the purifying electrodes, and to this end, the carbon is maintained in a state reactive to oxygen. I prefer to use carbon electrodes of small diameter and to operate them with sufficient current to make certain that oxygen liberated at the electrode surface will react with the carbon. The amount of current required depends on the temperature of the bath and the loss of heat by conduction through the electrodes and radiation.

It is also an object of the invention to provide an improved electrolytic cell and an improved arrangement of apparatus in combination therewith to remove oxygen compounds from a fused salt bath. In accordance with the apparatus of this invention, I provide purifying electrodes in the cell having a zone of electrolytic action and means for flowing the fused bath into the zone of electrolytic action to decompose the oxygen compound. One or more of the electrodes is a carbon anode which is maintained in such a condition that the oxygen of the decomposed compound combines with the carbon and forms an Oxy-carbon gas which escapes from the bath. I may so locate the purifying electrodes that any desired portion of the bath in the cell may be circulated into the zone of electrolytic action of the purifying electrodes.

I prefer to use carbon anodes of small diameter and employ refractory insulating sleeves to deflect the electric current to the lower ends or tips of the anodes. I adjust the electric current at an intensity suiiciently great to make certain that oxygen liberated at the anode surface will combine therewith forming oxygen carbon gas. I prefer to provide a very large cathode surface so that when said anode current flows there will be practically no cathode products liberated.

Although the use of direct current between the purifying electrodes is more certain of complete removal of the impurities, alternating current of low frequency may be employed. In the latter case, it is advantageous to have both electrodes of carbon and to use a high current density at both electrodes. A combination of alternating and direct current may also be used.

If, in any given electrolytie cell, the fused electrolyte rising from the anode and cathode mingle together at the upper part of the cell, the electrolyte which is purified by electrolysis with purifying electrodes may be drawn from any convenient place in the cell. But if there is an enclosed anode compartment with little or no opportunity for the electrolyte rising from the anode to mix with the electrolyte from the cathode compartment, it is best to draw the electrolyte from the anode compartment for purification since, under these conditions, the electrolyte in the anode compartment may accumulate a high oxygen content which affects the yields adversely and may have serious corroding action on the apparatus. Purification of the electrolyte taken from the cathode compartment alone will not improve these conditions.

In those cells which have an enclosed anode compartment and a sufficiently restricted passage for the return of electrolyte from the upper part of the anode compartment to the bottom of the anode, the upper surface of the electrolyte in the anode compartment standing somewhat above the upper surface of the bath outside the anode compartment, I provide a duct through which the electrolyte may flow from the anode compartment into the zone of electrolytic action of the purifying electrodes. I prefer to so arrange the duct that the electrolyte will flow under the action of gravity without the use of any kind of a pump, since the difference in head between the upper surfaces of the electrolyte inside and outside of the anode compartment may be made sufficient to circulate the electrolyte through the duct and past the purifying elec trodes. In an advantageous embodiment of my invention, I connect a duct to the feed compartment and circulate the electrolyte from the anode compartment through the feed compartment and then into the zone of the electrolytlc action of the purifying electrodes. Through a suitable opening or passage, the replenished electrolyte from the feed compartment which is purified in the zone of electrolytic action is passed into the bath.

In another arrangement of electrolytic cell embodying the invention, I may pass electrolyte from the cathode compartment through a duct as by means of a pump, for example an air-lift, into the feed compartment and then pass the replenished electrolyte from the feed compartment into the zone of electrolytic action of the purifying electrodes. I may also purify the electrolyte taken from the cathode compartment or from the anode compartment without rst passlng the electrolyte through the feed compartment.

It is to be understood that the purifying electrodes are not intended for the production of the metal being produced by the main electrodes and that they are designed, arranged and operated under conditions favoring the decomposition of oxygen compounds and the removal of the oxygen through its combination with carbon of the anode.

By way of illustration, I shall describe the invention as applied to the production of sodium and chlorine by the electrolysis of fused sodium chloride in a fused mixed bath of sodium chloride and calcium chloride.

Fig. 1 is a vertical sectional view taken through the center of a cell embodying the invention;

Fig. 2 is a horizontal sectional View from above along the line 2-2 of Fig. 1;

Fig. 3 is a partial vertical section taken along the line 3 3 of Fig. 2;

Fig. 4 is a partial vertical section taken radially at 4-4 of Fig. 2:

Fig. 5 shows a modification of the invention, and

Fig. 6 shows another modification of the invention.

The electrolytlc cell and associated apparatus of the invention illustrated in Figs. 1, 2, 3 and 4 comprise the usual container for a fused salt bath, a carbon anode, an annular cathode, a diaphragm, metal collecting hood, and an enclosed upper anode compartment wherein the chlorine gas is collected.

The anode has a hollow central opening l where it projects into the bath and a plurality of passages 2 connecting the central opening with the space 3 between the active surface 4 of the anode and diaphragm.

In the upper part of the cell, I provide an annular feed compartment E comprising an outer trough-like portion 1, an inner trough-like portion 8. the two being concentric and connected together by the opening S, and a cover lil. The end portion oi' the part 'l is connected by a duct i i with the upper anode compartment, the openlng into the anode compartment being covered with a shield l2 to prevent the passage of chlorine gas up into duct il. The portions of the feed compartment 'l and 8 are separated from each other by the annular double wall I3 which is filled with a thermal insulating material. I prefer to have the duct ll arranged to discharge fused bath from the anode compartment directly under the place where salt is deposited on the bath by the salt feed.

As best shown in Figs. 2, 3 and 4, I arrange in the inner part 8 of the feed compartment purifying electrodes l5 comprising two small graphite anodes I6. I use the adjacent wall portions of the feed compartment, anode compartment and the baille 35 as a cathode. The anodes are insulated from each other and from the cover by insulating bushings I8 where they pass through the cover I0 of the feed compartment and are supplied with direct current by the circuits I9 and 2li, and with alternating current by I mount the upper portion ol' the anodes inside the refractory sleeves or cells 22 which serve to direct the current to the lower tip of the anodes.

In the cell illustrated, the passages 2 and the space 3 are preferably so restricted that the bath in the anode compartment is held at the level 3B which is somewhat above the level 3l outside the feed compartment. This creates a head which forces the electrolyte from the anode compartment to flow through the duct Il into the feed compartment. The hot bath in the feed compartment mingles with the salt entering from the salt feed helping it to melt and the electrolyte flows in the direction of the arrows 32 through the opening S around the inner part of the feed compartmentl il in the direction of arrow 33 and through the slit 34 in the partition 35. The bath flows downward between the anodes I6 and the cathode which is the zone of electrolytic action of the purifying electrodes and the oxygen compounds which are dissolved in the b-ath are decomposed. The electric current between the electrodes is maintained sufficiently high that the oxygen set free combines with the carbon of the anodes and escapes from the bath as an Oxy-carbon gas. The replenished bath from the feed compartment from which the oxygen compounds have been removed in the zone of electrolytic action flows in the direction of the arrow 3B into the upper part of the cathode compartment. I prefer to use a scavenger in the feed compartment to combine vvith the oxygen therein and form oxygen compounds. I may use sodium, calcium, aluminum and other oxidizable substances as scavenger or mixtures such as sodium and calcium which may accumulate from other operations. I may advantageously collect the sodium which flows by the metal collecting hood and introduce it into the feed compartment as a scavenger.

In order to utilize globules of metal which escape the metal collecting hood and rise into the upper part of the cathode compartment as a scavenger in purifying the salt in the feed compartment, I provide an annular depending baille 4U which forms a pocket 4l into which the metal rises. The pocket has an opening 42 through which the metal flows into the feed compartment near the opening from the salt feed.

The sodium which flows into the feed compartment floats on the surface and is probably absorbed to a limited extent by the fused salts and combines with the oxygen. When other scavengers such as calcium or aluminum are used, the insoluble compounds settle in the feed compartment and tend to accumulate in the bottom 43 of the inner part 8 from which they may be removed from time to time by means of a ladle. The soluble oxygen compounds are carried along with the mixed bath and are decomposed in the zone of electrolytic action in the manner described. In treating mixed baths, I introduce from time to time small amounts of calcium chloride in the feed compartment through a` hole in the cover (not shown).

In the modif-led form of apparatus illustrated in Fig. 5, the outside part of the feed compartment 1 is provided with a pump 45 in the form of an air-lift by means of which the bath from the cathode compartment is pumped into the feed compartment by means of nitrogen or any suitable inert gas blown through the tube 46. I prefer to so arrange the pump 45 that the bath is dischargedV into the feed compartment in the immediate vicinity of the place where the dry sodium chloride is deposited. The cell constructed according to Fig. 5 may be operated in substantially the same manner as the cell of Fig. 1, with the exception that the bath does not flow under the action of gravity from the anode compartment into the feed compartment.

As shown in Fig. 6, I may arrange baliles 5i! and 5l across the inner portion 8 of the feed compartment at the bath level 3| and a baille 52 which extends upward almost to the level 3|. Baiie 52 directs the stream of bath upward into sweeping contact with the layer of sodium. It is advantageous to keep a. layer of molten sodium floating on the bath in the feed compartment between baffles 5D and 5l. Due to the heat transmitted through the bottom of the compartment, thermal convection currents cause all parts of the bath to rise up in contact with the layer of sodium and hence the bath is thoroughly cleansed of dissolved oxygen and oxygen combined with hydrogen in the form of Water.

I may add other scavengers such as calcium or aluminum to the bath in the feed compartment, in which cases the oxygen compounds formed are only partially soluble in the bath, and will be precipitated and will settle out at the oottom of the feed compartment and can be removed from time to time with a small ladle.

By removing the oxygen and oxygen-bearing impiulties from the cell continuously and preventing impurities from entering the electrolyte with replenishing materials, it is possible to maintain high ampere yields throughout the life of the cell, and to prevent to a considerable extent the destruction of the active anode surfaces by oxygen combustion or corrosion.

An advantage of this invention in the example described is that it is possible to maintain high yields with lower current density at the main anode since it is not necessary to use an intensity of electric current sufficient to maintain the lower part of the active surface of the anode reactive to oxygen and unnecessary to raise the intensity of current flow intermittently as described in my copending application now Patent No. 2,390,548, granted December 11, 1945.

I claim:

1. In the electrolysis of fused halide salt baths in electrolytic cells having an anode compartment separated in the upper part of the cell from the bath outside the anode compartment, a feed compartment at the upper surface of the bath and outside the anode compartment, and in which oxygen impurities are introduced into the electrolyte during operation of the cells, the improvement which comprises flowing bath containing oxygen from the anode compartment into the feed compartment, combining the oxygen with a metallic scavenger in the feed compartment to form an oxygen compound soluble in the bath, owing bath from the feed compartment into a zone of electrolytic action provided by purifying electrodes at least one anode of which contains carbon and is in a condition to combine with oxygen, decomposing the oxygen compound and combining the oxygen with the carbon of the said anode, subjecting the salt bath so treated for the decomposition of the oxygen compound to electrolytic decomposition by means of electrodes independent of the purifying electrodes.

2. The method of removing impurities in the form of oxygen introduced into the fused halide salt bath of an electrolytic cell during the operation thereof which comprises introducing into the bath a metallic scavenger for combination with the oxygen to form a bath-soluble oxygen compound, circulating the bath into a Zone of electrolytic action within the cell provided by purifying auxiliary electrodes at least one anode of which contains carbon, decomposing the oxygen compound and combining the oxygen with the carbon of the auxiliary electrode, removing the oxygen from the cell in the form of an oxycarbon gas and subjecting the salt bath so treated for the decomposition of the oxygen compound to electrolytic decomposition by means of electrodes independent of the purifying electrodes.

3. The method of removing oxygen introduced as an impurity into the fused halide salt bath of an electrolytic cell which comprises introducing a metallic scavenger into a portion of the bath from the anode compartment to form a bath-soluble oxygen compound with the oxygen, passing the bath containing the oxygen compound into a zone of electrolytic action provided by auxiliary purifying electrodes the anode of which is carbon to decompose the oxygen compound and form an Oxy-carbon gas which is removed from the bath, and subjecting the bath from the said electrolytic action to electrolytic decomposition in a zone of electrolytic action c7 which is independent of that provided by the purifying electrodes.

4. The method of removing impurities in the form of an oxygen compound introduced into the fused halide salt bath of an electrolytic cell during the operation thereof which comprises circulating a. portion of the bath from the cathode compartment into a feed compartment, introducing a scavenger metal into the salt in the feed compartment to combine with oxygen contained therein, passing the mixed bath from the feed compartment containing a compound formed by combination with the scavenger metal into a zone of electrolytic action provided by purifying electrodes the anode of which is carbon to decompose the oxygen compound contained there,- in and combining the oxygen from the decomposed oxygen compound wth the heated carbon anode to form an Oxy-carbon gas which is removed from the bath, and subjecting the bath from which the Oxy-carbon gas is removed to electrolytic decomposition in a zone of electrolytic 'action independent of that provided by the purifying electrodes. A

5.1Apparatus for the electrolysis of La-fused salt bath having an anode, a cathodel an anode compartment above the anode, a; feed compartment adjacent the anode compartment and a metal collecting hood,V the improvement which comprises baiiie. means in the upper portion of the .cell for collecting small globules of metal which escape collection in the metal collecting hood, passage means inoperative connection with the baille for passing the small particles of metal thus collected into the feed compartment to act as a scavenger in combination with oxygen, means for collecting and removing from the bath insoluble oxygencompounds formed from the said metal, and means for removing from the bath soluble oxygen compounds formed from said metal.

6. Apparatus according to claim 5 which comprises purifying electrodes and means for passing electrolyte from the feed compartment after introduction therein of the scavenging metal into contact with the purifying electrodes.

ROBERT J. MCNITT. 

